In many technical fields, a need exists for storing various liquid or gaseous media, such as compressed or liquefied gases, for extended periods of time and frequently at very high pressures. Many attempts have already been made in the past to satisfy this need by developing lightweight pressurized medium containers or pressure vessels that would accommodate the pressurized medium without suffering leakage losses or structural damage.
For a variety of reasons, not the least important of which is the relatively high ratio of pressure that the vessel walls are able to withstand to the weight of a vessel of a given capacity, it has been found advantageous to give such walls a multilayer or composite structure, including an inner liner and an outer shell surrounding the liner and in intimate contact therewith. The liner is formed of a material usually a metallic material that is compatible with (i.e. inert with respect to) and also completely or at least highly impermeable to the medium being stored.
All-metallic pressure vessels have been disclosed, for example, in U.S. Pat. Nos. 2,127,712; 2,661,113; 3,140,006; and 4,964,524, of which all but the second one are directed to vessels of multilayer construction. In this instance, one of the purposes of the liner is to form an inert protective barrier preventing the medium from reaching through gross leakage or permeation through the liner to the outer shell and possibly damaging the shell. However, due to their considerable thickness and intimate contact or engagement with the shell, the liners of all-metallic pressure vessels generally contribute significantly to the load bearing capacity of the vessel. In classical state of the art vessel fabrication, the liner represents a significant fraction of the total weight. Experience with such and similar all-metallic pressure vessel constructions has shown, on the other hand, that they are limited in applicability because they are either too heavy (a criterion that is of paramount importance for applications where weight is at a premium, such as in outer space applications), or expensive to manufacture, or prone to failure, especially due to metallic material fatigue at weakened or stress concentration regions after having been subjected to a number of pressurization and depressurization cycles.
With the advent and development of high strength filaments such as glass, graphite, and synthetic plastic material fibers, and of materials, such as epoxy resins, capable of forming a matrix embedding such filaments and bonding them together into a composite structure, attempts have been made, some more successful than others, to use such composite materials for the outer shell of the pressure vessel. Of course, due to the high strength-to-weight ratio of such materials, the overall weight of the resulting vessel is significantly reduced relative to that of a comparable all-metallic vessel of the same capacity and pressure rating. Examples of vessels of this kind are disclosed, for example, in U.S. Pat. Nos. 2,744,043; 2,827,195; 3,943,010; 3,969,812, 4,040,163; and 5,653,358.
For example, U.S. Pat. No. 5,653,358 among other elements, describes a tank of composite structure. A vessel is comprised principally of an inner liner (such as a metal liner) coated with a primer and an overwrap or jacket. To that end, the outer jacket is constructed, in a known manner, by superimposed and overlapping layers of impregnated filamentary material that contains glass, graphite or Kevlar™ fibers wrapped in different directions around the liner, with the interstices between the fibers or filaments being filled by impregnating material such as hardenable epoxy resin that, upon setting or hardening, forms a matrix that firmly embeds such fibers or filamentary material.
Thus, after hardening, the filamentary and impregnating material together form a composite, fiber reinforced solid body that is capable of withstanding most if not all of the forces applied to the vessel during its lifetime.
However, the prior art methods of applying the wrapped jacket to the metal liner of the vessel suffers from many drawbacks. In the prior art, a metal liner is first coated with a primer and then the adhesive is used to structurally couple the liner to the overlying filament wound composite. Most commonly a reticulating film adhesive is used. In such an application, pre-cut pattern shapes or gore panels are applied by gloved hands prior to the commencement of composite lay-up. In this application, fibers are impregnated with a wet winding resin before application. The bearing pressure for bondline curing, required to ensure a good bond between the adhesive and the metal liner, is developed as a by-product of the tension in the fiber tows (jacket material) that are filament wound in a pre-programmed repeating closure pattern over the liner and adhesive.
Upon completion of the winding, which consists of multiple layers of the reinforcing fibers and impregnating resin, the composite structure is cured. The adhesive and wet winding resins are compatible and co-curable.
This wet winding process by its nature develops variable bearing pressures on the liner as a result of fiber buildup near the polar regions (ie. boss and/or exit of the vessel) of the wind. This frequently results in roping/bridging of the fiber with a resulting loss of bearing pressure. The bearing pressure also varies due to resin rheology (time and temperature dependent viscosity response.) Resin trough behavior is recognized as resin bleedout during the early stages of wet winding cure (first stage also known as gelation). The variability in bearing pressure is a limiting factor in developing optimal adhesion of the overwrap layer to the metal liner. The critical parameter is adhesion to the metal liner, the adhesion to the co-cured overlying composite structure is readily achieved through selection of compatible and co-curable adhesives and wet winding epoxies.
It is understood, that structural coupling between the inner liner layer and the outer jacket layer is critical, particularly when the pressure vessels are filled with loads under high pressure. Poor adhesion between the overwrap layer and the metal liner caused by irregular bearing pressure during the overwrap application can result in liner elastic stability/buckling failure. The thin metal liner, as a standalone structural entity, is incapable of supporting the high bearing pressure imposed at zero or low pressure by the overlying composite after vessel autofrettage. The elastic stability or buckling failure of the liner results in dramatic reductions in fatigue life, resulting through cracks in the metal liner and leakage of contents within a small number of cycles.